1. Field of the Invention
The present invention relates to a device for measuring the thickness (caliper) of a sheet or web material. The latter can be, in particular, a moving material web and, for example, a moving fibrous web such as, in particular, a paper or board web.
2. Description of the Related Art
Previously known devices for measuring the thickness of paper are based on the three following measuring principles:    1. Contacting thickness sensor.            The current contacting thickness sensors are certainly very accurate. However, since they are in contact with the abrasive material web moving at high speed, which can be, for example, newsprint made of secondary material with CaCO3 filler, the contacting elements wear away very quickly. These sensors also frequently produce holes in the material web when the measuring sensors strike the web, and produce corresponding faults. In the case of matt coatings or high-gloss grades, the sensor sometimes produces markings on the web.            2. Air pad thickness sensor.            Air pad thickness sensors neither exhibit premature wear phenomena nor do they produce oles or other markings on the material web. However, they are not sufficiently accurate for newsprint or fine paper, which can be attributed to the instability with respect to the air pad thickness.            3. Laser thickness sensor using the laser triangulation method.            Laser thickness sensors operating on the basis of the triangulation method likewise exhibit no premature wear phenomena, nor do they produce holes or markings in or the sheet or the web. A laser measures the distance to the product web or the material web from both sides, and a magnetic sensor measures the thickness of the total measuring gap. The thickness of the material web is calculated by the laser measured values being subtracted from the magnetic measured value. The requisite repeatability for fine paper and newsprint lies in the range of about 0.25 μm. Even when there is no further source of error, a magnetic measurement for a 10 mm gap must have a repeatability of at least 1 in 40,000. The other error source includes the fact that, when the sheet to be measured is not exactly parallel with the gap, even a small misalignment between the upper and lower measuring head can entail a significant error. If the tangent of the angle between the sheet or the web and the measuring head is only 0.01, then the alignment error between he measuring heads must remain below 0.025 mm in order to achieve the 0.25 μm epeatability. The best scanning frames which can currently be obtained can maintain about 0.2 mm. The standard laser thickness measurement based on the triangulation method operates well only in the case of thick products but not in the case of lightweight, thinner sheets.        
What is needed in the art is a device for measuring the thickness of a material web which does not mark or damage the web, which does not wear out prematurely, and which is sufficiently accurate and precise.